If the ignition timing of a spark-ignition engine is too early, combustion occurs prematurely and the ascending piston is pushed down. On the other hand, if the ignition timing is too late, combustion occurs belatedly and the combustion pressure will act when the piston has descended by a large extent and no effective work is done.
Under these circumstances, active studies are being conducted to optimize the ignition timing of spark-ignition engines from various viewpoints such as improving the engine output and fuel economy and promoting the efficiency of rendering emissions harmless, thereby contributing to clean environments and energy conservation.
Theoretically, an optimum timing for igniting spark-ignition engines is defined as the point of time when the crank angle has reached a minimum spark advance for best torque (which is hereunder abbreviated as MBT for simplicity) and it is best to ignite at that point of time from the viewpoints of output and fuel economy. The term "minimum spark advance for best torque" (MBT) as used herein means the crank angle that corresponds to the ignition timing where a maximum torque can be produced without knock.
With conventional spark-ignition engines, the ignition timing is adjusted to MBT by first calculating the crank angle corresponding to MBT using various means and then controlling the ignition timing either mechanically or electronically. For example, JP-A-56-165772 discloses an apparatus for adjusting the ignition timing of an engine using a mixture of alcohol and gasoline as a fuel. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".) The apparatus comprises a sensor for detecting the concentration of alcohol in the mixed fuel and a control unit for adjusting the ignition timing on the basis of the output of the alcohol sensor. This apparatus will accelerate the ignition timing if the concentration of alcohol is above a preset level.
JP-A-1-193079 discloses an invention according to which the change in the rotational speed of an engine is regarded as a motion of uniform angular acceleration and the ignition timing is calculated on the basis of the period between two successive reference crank positions, whereby ignition is done at an optimal time that is associated with a specific state of engine running. JP-A-1-193079 also discloses a method of electronically controlling the timing of ignition of gasoline engines that enables ignition to be done at an optimal time that is associated with a specific state of engine running even if its rotational speed is varying.
However, the conventional methods for optimizing the timing of igniting spark-ignition engines are not completely satisfactory and have had various problems. For example, the apparatus for adjusting the ignition timing as disclosed in JP-A-56-165772, supra, has the limitation that it is only applicable to the case where the concentration of alcohol is near a specified or standard value and an optimal output cannot always be produced if fuels having sub-standard gasoline/alcohol concentrations are used. The method as disclosed in JP-A-1-193079, supra, has the problem that it is not necessarily capable of control for an optimal ignition timing if two fuels having different characteristics such as octane number or burning velocity are used, as exemplified by the case of using a gasoline of high octane number and a regular gasoline.
The present inventors conducted further studies and experiments on the basis of those prior art techniques and have finally found that an optimal ignition timing, or the ignition timing at which a spark-ignition engine produces a maximum thermal efficiency, is a linear function of a certain specified factor and independent of the kind of fuel, A/F ratio, air intake and the rotational speed of the engine.